SIO15-SS1 2020: Topic 7 Earthquake Seismology P and S Waves are - - PowerPoint PPT Presentation

SIO15-SS1 2020: Topic 7 Earthquake Seismology

SIO15-SS1 2020: Topic 7 Earthquake Seismology

P and S Waves are body waves

Image: S. Marshak “Earth, Portrait of a Planet”

SIO15-SS1 2020: Topic 7 Earthquake Seismology

SIO15-SS1 2020: Topic 7 Earthquake Seismology

• 1. P first, typically smallest signal
• 2. S, typically larger (felt by humans)
• 3. Love
• 4. Rayleigh, longest/largest wavetrain, most damaging

R

short video 7 on:

seismic waves how to locate an earthquake

p wave used in early warning systems warning time: up to 1 minute

SIO15-SS1 2020: Topic 7 Earthquake Seismology

there is already a fault why don’t blocks move all the time?

fault breccia/fault gouge: broken-off, ground up asperities asperities: protrusions that increase friction along a fault EQ: when stresses high enough to overcome friction

Friction Stress Friction Stress no EQ EQ!

• Fig. 5.18

SIO15-SS1 2020: Topic 7 Earthquake Seismology

• 1. stress builds up; asperities; no motion
• 2. stress overcomes friction -> EQ + aftershocks
• 3. slip along fault; stress drop
• start from 1)

fault locked very little creep fault locked very little creep

• Fig. 5.19

SIO15-SS1 2020: Topic 7 Earthquake Seismology

• some history of local seismicity in

Seattle/Tacoma area

• no recorded history of megathrust EQ
• 1700 orphan tsunami in Japan has no

corresponding local EQ -> MW=9.0 in Cascadia

• recurrence time ~ 300 years
• complex aseismic slip and episodic tremors

(GPS and seismometers)

• Fig. 5.21

SIO15-SS1 2020: Topic 7 Earthquake Seismology

• > 3.5 Mio per year
• small events often
• large events rare

SIO15-SS1 2020: Topic 7 Earthquake Seismology

Get recurrence time from # of EQ per year

E.g. 3 Mio EQ/year -> 1 EQ every 10.5s 0.3 EQ/year -> 1 EQ every 3.3 years

• Fig. 5.13

SIO15-SS1 2020: Topic 7 Earthquake Seismology

EQs not regular recurrence time has a certain probability < 100%

source: Abbott “Natural Disasters”

• Fig. 5.15

SIO15-SS1 2020: Topic 7 Earthquake Seismology

Probability of EQ before 2032

• 30% chance for a mag 7 quake before 2032,

for most places along SAF

• 90% chance for a mag 6 at Parkfield

recurrence time varies along same fault

prognosis for before 2004 seismic gap/recurrence

source: Abbott “Natural Disasters”

• Fig. 5.16

SIO15-SS1 2020: Topic 7 Earthquake Seismology

use seismic gap method

EQ recurrence time Parkfield EQs remarkably regular in > 100 years next predicted: 1993

28 September 2004, 10:15 PSDT * #7 late or #8 early???

• Fig. 5.17

SIO15-SS1 2020: Topic 7 Earthquake Seismology

• Mw= 6.5 December 22, 2003 San

Simeon EQ increased stress at Parkfield

• made #8 more likely???
• but why did #7 not happen?
• EQs can trigger/influence other EQs on nearby faults

(1992 Landers/Big Bear)

• very large EQs may trigger volcanism far away (Rayleigh waves!)

(2002 Denali EQ/Yellowstone; 2004 Sumatra/Alaska volcanoes)

• Fig. 5.17

SIO15-SS1 2020: Topic 7 Earthquake Seismology

InSAR: Interferometric Synthetic Aperture Radar

• overlay satellite altimetry map before and after EQ
• interferogram -> fringes show amount of slip along fault

Sep 17, 1999 Izmit, Turkey Earthquake

Image: NASA/JPL; wikipedia

• Fig. 6.43

SIO15-SS1 2020: Topic 7 Earthquake Seismology

• use stack of INSAR images to

get strain rate

• extrapolate to 250yrs

relaxed loaded

• example: southern SAF
• last major EQ: 250 years ago,

while north and middle SAF ruptured 1906, 1989 and 1857

• typical co-seismic slip for

big EQs: 7-10m

SIO15-SS1 2020: Topic 7 Earthquake Seismology

probability of strong shaking site amplification

• Fig. 6.44
• Fig. 6.45

takes into account local geology takes into account local geology, tectonics, seismicity

SIO15-SS1 2020: Topic 7 Earthquake Seismology

SCEC: Southern California Earthquake Center http:movie

• Fig. 6.46

SIO15-SS1 2020: Topic 7 Earthquake Seismology

* understand shaking characteristics of EQs * improve local forecasts for strength of shaking

Mw=7.2 El Mayor Easter (4 Apr) 2010

USGS Did-you-feel-it map

SIO15-SS1 2020: Topic 7 Earthquake Seismology

SIO15-SS1 2020: Topic 7 Earthquake Seismology

• don’t build, build to code, retrofit

(e.g. use wood, not bricks)

• construct lifelines to EQ building codes
• problematic in less developed countries
• grading according to code (landslides!)
• secure life lines/make disaster plans

What Government and People can do Got water? ...flashlight canned food batteries battery-op radio

• Fig. 6.42

SIO15-SS1 2020: Topic 7 Earthquake Seismology

SIO15-SS1 2020: Topic 7 Earthquake Seismology

We just don’t know! ……yet….. forecasting weather is much easier!

SIO15-SS1 2020: Topic 7 Earthquake Seismology

SIO15-SS1 2020: Topic 7 Earthquake Seismology

So, when is it coming?